In typical crystal growing apparatus of the type wherein a crystal of substantially cylindrical shape is pulled from the melt, one typical pulling mechanism includes a rotatably mounted and vertically slideable pull rod, to the lower end of which a crystal seed is attached. During the process of growing the desired crystal, the rod is vertically translated at the desired rate at which the crystal is drawn from the melt. The disadvantage of such a rod-supported pulling mechanism is that a support structure for the mechanism rises to a height which includes the length of the pull rod, the mounting chuck for the crystal seed and the desired length of the crystal to be pulled from the melt. Because it is economical and practical to grow crystal cylinders or ingots of a length in excess of one meter, it becomes desirable to move the pull rod vertically through considerable distances. It is apparent that the length of a crystal grown by an apparatus of this type is likely to be limited by a reasonable limit on the height of the towering overhead structure of such crystal growing apparatus.
A relatively recent development in the crystal growing apparatus art uses an overhead pulling mechanism, wherein the crystal seed is attached to an end of what is referred to as a wire rope, and which is in essence a stranded, flexible cable. Under the weight of a seed holder and of the additional weight of the grown crystal, the cable remains in tension to support the growing crystal relative to the melt. In an upper portion of a growing chamber, the cable is reeled onto a drum, thereby eliminating the additional height in the structure of the chamber to house a rigid rod. The drum is mounted to revolve about the downward extended length of the cable as an axis such that the crystal can be rotated on a vertical axis with respect to the apparatus.
While such cable-supported pulling mechanism overcomes disadvantages which may be related to a solid rod pulling mechanism such that the cable-supported mechanism should therefore permit the growth of a relatively long crystal ingot, a disadvantageous tendency has been observed in some cable-supported crystal pulling mechanisms. A problem of oscillation tends to develop as the growing crystal is pulled from the melt by the crystal pulling mechanism, whereby the crystal starts to swing perpendicular to its axis of rotation and thereby away from a central axis of the apparatus. Such oscillation at times results in undesirably non-uniform growth of the crystal, and has already resulted in lateral oscillatory displacements of the crystal of such magnitude that the crystal growing process had to be terminated prematurely.
It is, therefore, desirable to support the growing crystal by a mechanism which does not require added space in a growing chamber for housing a rigid support rod in addition to the grown crystal, but which also does not tend to tolerate or to support oscillations of the grown crystal away from the intended longitudinal axis of rotation of the crystal.